Steering mechanism of a vehicle, comprising a variable centering device

ABSTRACT

A vehicle steering system having a steering handle for steering at least one wheel is provided, and includes an arrangement for applying torque to the steering handle and a sensor for detecting applied steering torque. The arrangement generates a return torque, as a function of the steering angle, for driving the steering handle into a central position. During a first driving state in which applied steering torque is above a threshold value, the function curve of the return torque increases monotonically and no return torque is generated at a steering angle of zero. During a second driving state in which applied steering torque is below the threshold value, the arrangement generates a return torque that is greater than that given by the function curve in the first driving state, at least at small steering angles.

The present invention relates to a vehicle steering system having asteering handle for steering at least one wheel, the steering systemhaving an arrangement for applying a torque to the steering handle and asensor for detecting the steering torque T_(driver) applied to thesteering handle by the person steering the vehicle, the arrangementgenerating a return torque T_(return) which is a function F(φ_(steer))of the steering angle φ_(steer), and the return torque T_(return)driving the steering handle to a central position (φ_(steer)=0°).

The return of the vehicle steering system restores a vehicle that hasbeen turned to straight-line travel. Exact automatic centering by thevehicle steering system itself is also desirable when, inter alia, theperson steering the vehicle is not touching the steering wheel or isonly applying a very small force to it.

When a vehicle steering system performing an active return is beingproduced, there are, as things stand at the moment with regard toproduction, always two conflicting aims which arise relating to thegeneration of the return torque. On the one hand the vehicle steeringsystem must have good centering with the residual angle being onlyminimal and on the other hand it must transmit a harmonized feel fromthe steering, particularly when steering takes place through the centralposition.

To obtain very good centering with the residual angle being onlyminimal, a prerequisite is a function curve for the restoring force suchas is shown by the dashed curve in FIG. 1. In this case, a return torqueor force T_(return) is generated down to a steering-wheel angle of 0°(zero degrees), which means that the steered wheels are adjusted to thecentral position even when travelling slowly. However, due to thediscontinuity in the path followed by the curve, the feel given by thesteering is unpleasant when steering through the center if the personsteering the vehicle has his hands on the steering handle or steeringwheel.

However, a harmonized feel from the steering when steering through thezero position (central position) can only be obtained with a torquecurve f₁((φ_(steer)) for the return torque T_(return) of the kind whichis indicated by the solid curve in FIG. 1. However, exact centering isnot obtained with a return torque T_(return) of this kind because, dueto the decline in the return torque, there is still a residual angle,particularly at low vehicle speeds and/or when there is a high opposingtorque due to friction.

The object underlying the invention is therefore to provide a vehiclesteering system in which there is a harmonized feel from the steeringwith, at the same time, exact centering even at low vehicle speeds.

This object is achieved by a steering system having the features givenin claim 1. Further advantageous embodiments of the invention areproduced by the features given in the subclaims.

What is advantageously achieved by means of the invention is excellentcentering with, at the same time, a harmonized feeling from the steeringwhen steering through the central position. This is achieved by virtueof the fact that, when the vehicle is in different driving states oroperating states, return torques T_(return) of different sizes arepreset or adjusted to by the vehicle steering system. In this way, atorque curve f₁(φ_(steer)) of the kind shown by the solid line in FIG. 1is always preset when the person steering the vehicle applies a manualtorque of more than a threshold value T_(threshold) to the steeringwheel. This is generally the case when there is some active interventionby the driver on the steering. As soon as the manual torque drops belowthe threshold value, i.e. if the driver releases the steering wheel,then, in the simplest case, a curve of, for example, the kind shown bythe dashed curve in FIG. 1 is preset for the return torque. However,within the scope of the invention, it is possible for any desired shapeof curve or even any other desired functions to be preset or selectedfor the return torque in the second driving state. It merely needs to beensured that a sufficiently large return force, i.e. a sufficientlylarge return torque, is generated, particularly at small steeringangles, for the wheels which have been steered away from the centralposition to be moved to it.

Depending on the speed of the vehicle, forces of different sizes arerequired to move the wheels which have been steered away from thecentral position to be moved back to it. In this way, the forcesrequired for the purpose are greater at low vehicle speeds than athigher vehicle speeds. It is therefore likewise within the scope of theinvention for the return torque which is preset or generated to bedependent, at equal steering angles, on the speed of the vehicle in thesecond driving or operating state.

In all the possible embodiments, it is possible in this case for thethreshold value for the torque to be a constant value. It is howeverequally possible for the threshold for the torque, by which the twodriving states which are essential to the invention are distinguishedfrom one another, to vary with the speed of the vehicle.

In an embodiment of the simplest kind, as has already been described andas is shown in FIG. 2, a return torque curve f₁(φ_(steer)) of the kindshown by the solid line in FIG. 2 is assigned to the first drivingstate. The return torque curve assigned to the second driving state,i.e. where a manual torque is below the threshold value, is of the kindshown by the dashed line in FIG. 2. The box indicates that a distinctionis made between the two driving states only at steering angles φ_(steer)which are smaller than φ_(threshold). The distinction between thedriving states need not be dependent on φ_(threshold) however.

In a second possible embodiment, the first driving state is likewiseassigned a return torque curve f₁(φ_(steer)) of the kind shown by thesolid line in FIG. 1, whereby a harmonized feel is produced for andconveyed by the steering when the zero position of the steering angleφ_(steer) is steered through. The return torque curve f₁(φ_(steer))assigned to the second driving state, i.e. where a manual torque isbelow the threshold value, is of the kind shown by the solid line inFIG. 1, but an additional torque T_(int) is added to this torque curve.This additional torque T_(int) may be of a constant value. It is howeverequally possible for the additional torque T_(int) to be a function ofthe steering angle and/or of the speed of the vehicle.

The return torque that is obtained for the two driving states is thus asfollows:T _(return) =f ₁(φ_(steer))  Driving state IT _(return) =f ₂(φ_(steer))=f ₁(φ_(steer))+T _(int), orT _(return) =f ₁(φ_(steer))+T _(int)(φ_(steer)) orT _(return) =f ₁(φ_(steer))+T _(int)(φ_(steer) , V _(vehide)).  Drivingstate II

In a particularly advantageous embodiment, the additional torque T_(int)is calculated by integrating the steering angle over time.

In theory, there are thus a plurality of possible ways of calculatingT_(int).

In the simplest case, the steering angle is integrated over time:T _(int=) m∫φ _(steer) dt.

If the steering angle remains the same, the additional torque T_(int)thus becomes greater with time. So that it does not become too great, itis sensible for T_(int) to be limited to a maximum value T_(int,max).T_(int,max) may depend in this case on speed and/or on the steeringangle. In this integration, the rate of integration is dependent only onthe constant m.

It has been found that more comfortable steering can be obtained if therate of integration also depends on the speed of the vehicle. In thisway, a drive of excellent comfort can be obtained by using integrationas follows:T _(int) =∫k(v _(vehicle))φ_(steer) dt

In principle, the function k(v_(vehide)) can be selected to be asdesired in this case. However, it has been found useful for the rate ofintegration to increase as the speed of the vehicle increases. Byselecting the rate of integration in this way, it is ensured that noadditional return torque T_(int), or one which is only negligibly small,is generated when the zero position is steered through quickly. Anadditional return torque T_(int) is only ever generated when thesteering wheel is in the region of the zero position and the driver doesnot have his hands on it. FIG. 3 is a flow chart for the last embodimentelucidated.

With all the embodiments which are described in the present patentapplication, it is possible for the two driving states to bedistinguished, on the basis of the manual torque applied to the steeringhandle or steering wheel, only at small steering angles, and for onlyone return torque curve f₁(φ_(steer)) to exist for all manual torqueswhen the steering angle φ_(steer) is above a threshold valueφ_(threshold). Advantageously, any integration is stopped in this way atsmall steering angles (which are wanted by the driver). The integratortherefore responds only if the torque T_(driver) applied by the driveris below a certain value T_(threshold) and the steering angle φ_(steer)is below a threshold value φ_(threshold).

With all the embodiments described above, it is also possible for theresidual angle φ_(threshold) too to depend on the speed of the vehicleV_(vehicle). It is therefore important for good driving comfort for theresidual angle φ_(threshold) to become smaller as the speed of thevehicle increases.

1-10. (canceled)
 11. A vehicle steering system having a steering handlefor steering at least one wheel, comprising: an arrangement for applyingtorque to said steering handle, and a sensor for detecting a steeringtorque applied to said steering handle by a person steering saidvehicle, wherein said arrangement generates a return torque, which is afunction of the steering angle, and wherein said return torque drivessaid steering angle into a central position, wherein during a firstdriving state in which an applied steering torque is above a thresholdvalue, a function curve of said return torque increases monotonicallyand no return torque is generated at a steering angle of zero degrees,and wherein during a second driving state in which an applied steeringtorque is below the threshold value, said arrangement generates a returntorque that is greater than that given by said function curve of saidreturn torque in said first driving state, at least at small steeringangles.
 12. A vehicle steering system according to claim 11, wherein afunction curve of said return torque is a function of vehicle speedduring said second driving state.
 13. A vehicle steering systemaccording to claim 11, wherein a resulting return torque for said seconddriving state is composed or calculated from a sum of said functioncurve for said return torque and an additional torque.
 14. A vehiclesteering system according to claim 13, wherein said additional torque isa function of at least one of vehicle speed and steering angle.
 15. Avehicle steering system according to claim 13, wherein said additionaltorque is calculated from the intergralT_(int) =m·φ _(steer) dt wherein φ_(steer) is the steering angle, and mis a constant.
 16. A vehicle steering system according to claim 13,wherein said additional torque is calculated from the integralT _(int) =∫k(v _(vehicle))φ_(steer) dt Where the φ_(steer) is thesteering angle, and k is a speed-dependent constant or is a function ofthe vehicle speed v_(vehicle).
 17. A vehicle steering system accordingto claim 16, wherein the rate of integration is controlled by k.
 18. Avehicle steering system according to claim 16, wherein the function k(v_(vehicle)) increases monotonically.
 19. A vehicle steering systemaccording to claim 13, wherein said additional torque is limited to amaximum value.
 20. A vehicle steering system according to claim 19,wherein said maximum value of said additional torque is a function ofthe speed of said vehicle.